On the occasion corresponding to this four years ago all thoughts were anxiously directed to an impending civil war. All dreaded it, all sought to avert it. While the inaugural address was being delivered from this place, devoted altogether to saving the Union without war, insurgent agents were in the city seeking to destroy it without war—seeking to dissolve the Union and divide effects by negotiation. Both parties deprecated war, but one of them would make war rather than let the nation survive, and the other would accept war rather than let it perish, and the war came.
Lincoln reminds the audience of the occasion four years previous. He makes it clear that the south instigated the hostilities. The third sentence points out that the "insurgent agents" were busy with their proposition of dissolving the Union - a proposition that Lincoln as duty-bound to abjure. The fourth sentence repudiates any notion of an acknowledgement of personal fault (mea culpa).
I think that the more important lesson here was Lincoln's spirit of reconciliation. He could have easily used the occasion for a "victory lap" but instead he was thoughtful, respectful, and humble.
With malice toward none, with charity for all, with firmness in the right as God gives us to see the right, let us strive on to finish the work we are in, to bind up the nation's wounds, to care for him who shall have borne the battle and for his widow and his orphan, to do all which may achieve and cherish a just and lasting peace among ourselves and with all nations.
A not so clever way of saying a peace delegation wanted to negotiate. Slimy politicians are not a new phenomenon.